Can I just enter my passkey into the v6 gpu clients and qualify for the bonus ?

Yes, and that's probably what you should do. Continue folding regular projects and wait patiently for QRB to complete beta testing. If you start from scratch, though, the client will still fetch the new sucky core. You have to use an existing folding directory that already has a copy of the old v2.22 core in there and to make sure you fold only WUs that do not force the new core (i.e., regular units, not -beta or -advanced).

Kepler, incidentally, needs the new core. Everything else would be better off without it. Eventually, the -beta and -advanced projects will be released full-fah, though, in which case the v2.25 core is coming for us all, whether we like it or not. Hopefully QRB will be ready by then, offsetting the reduced performance of the core with increased rewards per unit.